cli/show: use decryption policy "auto" by default.

When showing a message, if the user doesn't specify --decrypt= at all,
but a stashed session key is known to notmuch, notmuch should just go
ahead and try to decrypt the message with the session key (without
bothering the user for access to their asymmetric secret key).

The user can disable this at the command line with --decrypt=false if
they really don't want to look at the e-mail that they've asked
notmuch to show them.

and of course, "notmuch show --decrypt" still works for accessing the
user's secret keys if necessary.